In recent years, a technique by which a thin film transistor (TFT) is manufactured using a semiconductor thin film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices and urgently developed particularly as switching elements in image display devices.
Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. For example, metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like, and thin film transistors in each of which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (see Patent Documents 1 to 4, Non-Patent Document 1).
Further, not only single-component oxides but also multi-component oxides are known as metal oxides. For example, InGaO3(ZnO)m (m: natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (see Non-Patent Documents 2 to 4).
Further, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (see Patent Document 5, Non-Patent Documents 5 and 6).